As complementary metal-oxide-semiconductor (CMOS) devices are scaling down, the leakage current is increased, and the device performance is degraded, due to increased short channel effects. New devices are needed to overcome these problems without requiring new materials and process steps.
The tunneling field effect transistor (TFET) is a quantum mechanical device that shows reduced short channel effects and smaller leakage currents compared to standard complementary metal-oxide-semiconductor (CMOS) devices. FIG. 1 shows an exemplary n-channel TFET 10 that is located in a p-well formed in a semiconductor substrate 12. Specifically, the n-channel TFET comprises a p-doped source region 22 and an n-doped drain region 24, which are spaced apart from each other by a channel region 23. A gate stack that comprises a gate dielectric 26 and a gate conductor 28 is located over the channel region 23.
The structure of the TFET is based on a metal-oxide-semiconductor (MOS) gated pin-diode. In the off-state, the TFET works as a low-leakage reverse biased junction diode between the source and drain regions 22 and 24. The barrier built by the reverse biased diode is larger compared to the barrier of MOSFET. This results in reduced sub-threshold leakage and suppressed direct tunneling even at very short channel lengths. When a bias is applied to the gate conductor 28, an electron channel is induced in the channel region 23. Once the channel charge concentration is degenerated, a tunneling junction is formed between the channel region 23 and the source region 22, through which the tunneling current flows.
As the TFET is scaling down from the 130 nm to the 90 nm node, use of steeper source doping gradients in the TFET has shown significant advantages in improving the performance of the tunneling junction. With the steeper source doping gradients, the on-current of the 90 nm node TFET is no longer limited by the tunneling junction only, but by the combination of the Zener diode and the MOS-channel. In the 65 nm node TFET, use of even steeper source doping gradients showed an on-current only limited by the MOS-channel. See Th. Nirschl et al., “The Tunneling Field Effect Transistor (TFET) as an Add-on for Ultra-Low-Voltage Analog and Digital Processes,” IEDM (2004), pp. 195-198; see also Th. Nirschl et al., “The 65 nm Tunneling Field Effect Transistor (TFET) 0.68 μm2 6 T Memory Cell and Multi-Vth Device,” Proceedings of ESSDERC (Grenoble, France 2005), pp. 173-176.
There is a continuing need for improved TFET structures with further enhanced tunneling currents.